I've been on about concurrency orientated programming languages for a while now and mostly I've been working in Python; because I like Python. But I keep hitting the edges of the language. Generators were a very promising feature before I knew what they actually were. When they were being discussed it looked like Python was going to get full coroutines, but in the end generators ended up being crippled in several ways:

You cannot yield inside a function call because, once you try to, that function then becomes a generator too. This is the big problem.

You can only pass values out of a generator, not the other way round. This may be addressed in Python 2.5 with PEP 342. You can already get around it to some degree by updating an external variable.

I'm sure generators solved the needs of some percentage of users at a lesser complexity and runtime cost of full coroutines. But support for PEP 342 is already showing that they struck the balance too far to the side of minimal changes.

But the good news is that someone is building Erlang like concurrency primitives with Gambit in the form of a project called Termite. Gambit is a Scheme which can compile to C code (as well as being interpreted) and has support for full lightweight threads and continuations. If you read the linked slides above by Joe Armstrong you'll see his challenge to language writers about the number of message passing threads in a ring. That challenge is well met by Gambit in the examples directory.

Termite doesn't have a source code release yet, but it should be soon. And scheme certainly has all the power one could ever want (and a syntax that no one would want). I'll post further comment when Termite is real.